1. Field of the Invention
The present invention relates to the sheet for sealing a solar cell between a front and a back material made of a sheet or a plate of a glass, a plastic and the like, particularly relates to a non-crosslinked solar cell sealing sheet capable of performing thermal lamination at low temperature.
2. Description of the Related Art
Conventionally, as a solar cell sealing sheet of this type, an organic peroxide-containing ethylene-vinyl acetate copolymer (abbreviated as EVA) has been generally used, since it has resin characteristics of good flexibility, high transparency, and excellent long-term durability by blending with an appropriate additive such as a weathering stabilizer, an adhesion promoter and the like.
However, because EVA has such drawbacks as lower melting point and poorer heat resistance that causes heat distortion at the temperature where a solar cell module is used, the heat-resistance has been effected by blending it with an organic peroxide in order to form a crosslinked structure.
A solar cell sealing sheet can be formed by a publicly-known sheet forming method capable of forming a polyolefin, but such a method as blending the organic peroxide has problems to cause deterioration of high speed productivity since the sheet formation is forced to be performed at low temperature in order to prevent decomposition of the organic peroxide.
In the process for producing a solar cell having the structure of (a surface protection layer=glass, plastics)/(a solar cell sealing sheet)/(a power module=a solar cell element)/(a solar cell sealing sheet)/(a surface protection layer=glass, plastics), generally employed are two processes composed of a tentative adhesion process by thermal lamination under vacuum and a crosslinking process in an oven at high temperature. This crosslinking process by organic peroxide requires several ten minutes, and therefore shortening of the crosslinking process time or elimination itself is strongly requested.
Further, there exists the concern that the power generation efficiency may be lowered by the adverse effect on a power module caused by a decomposition gas (acetic acid gas) from EVA or by the vinyl acetate group of EVA itself.
In order to avoid the above-mentioned problems associated with EVA, a solar cell sealing sheet employing an ethylene-•-olefin copolymer (Japanese Patent Laid-Open Publication No. 2000-91611) was proposed. It has been considered that these materials may reduce the adverse effects on a power module, but the balance between heat resistance and flexibility was not sufficiently good, and in addition, good heat resistance could not be realized without crosslinking, so that it has been difficult to eliminate the crosslinking process.
Inventors of the present invention have tackled the development of a solar cell sealing sheet usable even without being crosslinked, based on polypropylene or a copolymer mainly composed of propylene having excellent heat resistance. As a result of investigation by the present inventors, it has become clear that, although the solar cell sealing sheet based on such polypropylene or a copolymer mainly composed of propylene has good flexibility and heat resistance, there exist problems that the applicable temperature range for thermal lamination (process to adhere or fuse by heating a stack of a solar cell power module, a solar cell sealing sheet of the present invention, and further a glass or a back sheet) during solar cell manufacturing is very narrow.
During thermal lamination, low temperature as possible (specifically 160° C. or less, or more preferably below 160° C.) is preferred for thermal lamination, since a power module constituting the above-mentioned solar cell and a surface protection layer are damaged at high temperature.
In Japanese Patent Laid-Open Publication No. H11-163377, thermal lamination of a transparent resin between 120° C. and 160° C. is also described.